Cornea demands oxygen. Thus, oxygen has to be supplied to the cornea via the contact lens when the contact lens is placed on the cornea. Forms of supplying oxygen to the cornea vary dependent upon a kind of the contact lens. That is, a hydrous contact lens and non-hydrous contact lens differs from each other in the oxygen supplying form.
The foregoing non-hydrous contact lens, in general, is known as a hard contact lens. The hard contact lens is configured of, for example, silicone system resin or fluorine system resin. The reason is that the silicone resin and the fluoride resin have high oxygen permeability. The oxygen-permeable hard contact lens of this type is proposed, for example, in Japanese Patent Publication No. 63-36646 (1988) and Japanese Patent Publication No. 62-55122 (1987). However, the hard contact lens is poor in the wearing feeling. In particular, it causes severe pain when dust enters an eye.
The foregoing hydrous contact lens, in general, is known as a soft contact lens. The soft contact lens is soft as compared with the hard contact lens. Thus, the soft contact lens is excellent in the wearing feeling. The hydrous soft contact lens is remarkably prevailing from such a reason. With the case of the hydrous soft contact lens, oxygen dissolved into water is supplied to the cornea through the foregoing lens. Thus, with the case of the hydrous soft contact lens, it is preferable that a water content rate of the foregoing lens is high. That is, the high water content rate means that the oxygen supplying amount (oxygen permeating amount) is much. The water content rate higher than 70% causes the mechanical strength of the contact lens to decline. For examples, it results in being easily torn. Thus, it cannot be said safely that it is enough to increase the water content rate.
The silicone hydrogel contact lens incorporating the silicone in order to enhance the oxygen permeability is proposed (U.S. Pat. No. 5,486,579, and Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 1995-505169). However, the silicone compound and hydrophilic compound are poorly miscible with each other. For this, homogeneous polymerization is difficult. Thus, many leachable substances are detected. In addition, rubber elasticity peculiar to the silicone is yielded and flexibility is lost. For this, the maximum feature of the soft contact lens (the excellent wearing feeling) cannot be obtained. Further, the silicone is strongly hydrophobic. For this, water wettability of the lens surface is poor. The wearing feeling is not good from this viewpoint. Further, protein and fat easily adhere thereto. For this, the silicone hydrogel contact lens necessitates a surface treatment. However, durability of the surface treatment is not good.
On the other hand, nowadays, disposable contact lenses have prevailed from a viewpoint of handling of the contact lenses. That is, the disposable contact lenses on the premise that a replacement schedule is short (one day, one week, two weeks, or one month) have prevailed. An increase in the number of such disposable contact lenses incurs an anxiety over environmental destruction by the contact lenses made of plastics having oil resources as a material that are hardly decomposed. Thus, for example, the contact lenses are preferably manufactured from plant resources (plant-based polymer materials).
A development for new materials of the hydrous contact lens has been promoted from such a background. In particular, a development for the plant-based polymer materials that are highly hydrous, is excellent in the oxygen permeability, yet is rich in the mechanical strength, is excellent in the water wettability in addition hereto, is good in the wearing feeling, and is also high in the safety for human body has been promoted.
For example, the hydrous soft contact lens having cellulose as a raw material is proposed (JP-P1996-184786A).